You can get in real close, say under 1000 yds when conditions are right ie. overcast, no moon, choppy water ect. while keeping a minimal aspect to them.
BUT I've noticed that when my torps hit (or near- miss) that my boat gets peppered with gunfire almost immediately
. You can sneak in, but make sure that you kill your target quick and know that anything else around is gonna spot you and have a field day, lol.